ProjectManager: New File or Project... when having a project open

More or less a follow-up to QTCREATORBUG-17994 I'd say..

Triggering "New File or Project..." and choosing any project now sets the last wizard page to add the project as a subproject to an existing already opened one (if possible - e.g. having a subdirs project opened).

This had to be done manually up to QC 4.2.x or by explicitly triggering "Add New..." on the project node (which set the project related to the node on the last page) - but triggering "New File or Project..." normally had set the "parent project" to None by default.